1. Field of the Invention
The present invention relates to an optical head for optically reading and/or writing data from/on an optical data storage medium (such as an optical disc) including multiple data storage layers and also relates to an optical data processing apparatus including such an optical head.
2. Description of the Related Art
Various techniques have been developed in order to increase the storage capacity of an optical disc in one way or another. For example, as some people proposed it, the storage density or capacity of an optical disc is increased either by shortening the wavelength of a laser beam for use to read and write data from/on the disc or by increasing the numerical aperture (NA) of an objective lens for focusing the laser beam on the disc. Others proposed that the storage capacity be increased by providing multiple data storage layers, on which data can be written, for a single optical disc. In this technique, the storage capacity of an optical disc can be increased without changing the wavelength of a laser light source or the numerical aperture of an objective lens to be built in an optical head. And this technique has already been applied to a digital versatile disc (DVD) and other storage media. For instance, a single-sided two-layered DVD includes two data storage layers, from each of which data can be read by being irradiated with a laser beam coming from the same direction.
In reading and/or writing data from/on an optical disc including two or more data storage layers to increase the overall storage capacity, the optical disc drive needs to perform a process of sensing on what data storage layer the laser beam is currently focused by the objective lens (i.e., what is called a “layer sensing process”). Japanese Laid-Open Publication No. 9-259456 discloses how such a layer sensing process works in the prior art. Hereinafter, the conventional layer sensing process will be described with reference to FIGS. 19 through 22.
FIG. 19 shows a schematic configuration for members of an optical head 100 to carry out the layer sensing process. The optical disc 107 illustrated in FIG. 19 includes two data storage layers (i.e., Layer 0 and Layer 1). In this description, one data storage layer, which is more distant from an objective lens 106 of the optical head 100 (i.e., the deeper data storage layer), will be referred to herein as “Layer 0”, while the other data storage layer, which is closer to the objective lens 106 (i.e., the shallower data storage layer), will be referred to herein as “Layer 1”.
If a semiconductor laser diode 101 has radiated a linearly polarized laser beam, then a diffraction grating 102 produces three beams, consisting of a main beam and two sub-beams sandwiching the main beam, from the laser beam. Those beams are then transformed by a collimator lens 103 into a substantially parallel light beam, which is reflected by a beam splitter 104 and then transformed into a circularly polarized light beam by a quarter wave plate 105. Subsequently, the circularly polarized light beam is focused by the objective lens 106 on either Layer 0 or Layer 1 of the optical disc 107 by way of its substrate. It should be noted that the diffraction grating 102 is adjusted so as to perform a tracking control operation by a three-beam method.
Next, the laser beam is reflected by either Layer 0 or Layer 1 of the optical disc 107, transmitted through the substrate and objective lens 106 again, and then transformed by the quarter wave plate 105 into a different linearly polarized light beam from the previous one, which is subsequently transmitted through the beam splitter 104 and then focused by a detector lens 108 onto a light detector 10. The light detector 110 includes a number of photodetectors, each of which generates and outputs a light quantity signal having a level that represents the quantity of the light received there.
FIG. 20 shows an arrangement of five photodetectors 110a, 110b, 110c, 110d and 110e provided for the light detector 110. The light detector 110 includes a quadrant photodetector 110a for receiving the main beam, two photodetectors 110b and 110c for receiving the sub-beams to detect a tracking error signal, and two more photodetectors 110d and 110e for receiving the sub-beams for layer sensing purposes. The main beam of the laser beam, which has been reflected from a predetermined data storage layer of the optical disc 107, forms a beam spot m on the photodetector 110a, while the two sub-beams thereof form beam spots s on the photodetectors 110b and 110c, respectively. As shown in FIG. 20, the photodetector 110b, quadrant photodetector 110a, and photodetector 110c are arranged in line in this order from left to right. The photodetector 110d is provided between the photodetector 110b and quadrant photodetector 110a, while the other photodetector 110e is provided on the opposite side of the photodetector 110c (i.e., so as not to face the quadrant photodetector 110a).
FIG. 21 shows where the respective reflected beams are incident on the light detector 110 when the laser beam is focused by the objective lens on Layer 0 of the optical disc 107. As shown in FIG. 21, the main beam reflected from Layer 0 forms a beam spot m0 approximately at the center of the photodetector 110a, while the sub-beams reflected from Layer 0 form beam spots s0 approximately at the respective centers of the photodetectors 110b and 110c. At the same time, the laser beam reflected from the other Layer 1 also forms bigger beam spots m1 and s1 with lower intensities. It should be noted that the center of the beam spot m1 formed by the main beam that has been reflected from Layer 1 substantially matches that of the beam spot m0 formed by the main beam that has been reflected from Layer 0. On the other hand, the center of each beam spot s1 formed by the sub-beam that has been reflected from Layer 1 significantly shifts from that of its associated beam spot s0 formed by the sub-beam that has been reflected from Layer 0.
FIG. 22 shows where the respective reflected beams are incident on the light detector 110 when the laser beam is focused by the objective lens on Layer 1 of the optical disc 107. As shown in FIG. 22, the main beam reflected from Layer 1 forms a beam spot m1 approximately at the center of the photodetector 110a, while the sub-beams reflected from Layer 1 form beam spots s1 approximately at the respective centers of the photodetectors 110b and 110c. At the same time, the laser beam reflected from the other Layer 0 also forms bigger beam spots m0 and s0 with lower intensities. It should be noted that the center of the beam spot m0 formed by the main beam that has been reflected from Layer 0 substantially matches that of the beam spot m1 formed by the main beam that has been reflected from Layer 1. On the other hand, the center of each beam spot s0 formed by the sub-beam that has been reflected from Layer 0 slightly shifts from that of its associated beam spot s1 formed by the sub-beam that has been reflected from Layer 1.
In this case, the layer sensing signal RD is given by:RD=(output light quantity signal of photodetector 110d)−(output light quantity signal of photodetector 110e)
For example, if RD>0, then it can be sensed that beam spot is formed on Layer 0. On the other hand, if RD<0, then it can be sensed that beam spot is formed on Layer 1.
However, this layer sensing method is supposed to use sub-beams by the three-beam method, and cannot be applied to an optical head that adopts a one-beam method using no sub-beams or to an optical drive including such an optical head. The three-beam method is much more complicated in configuration and processing than the one-beam method. Thus, there is a growing demand for a layer sensing process that adopts the simpler one-beam method. Also, according to the three-beam method, the sub-beams have such small light quantities that the beam spots to be detected have just low intensities and relatively broad areas. Thus, it is difficult to achieve a sufficiently high SNR in such a situation.